Human natural killer (NK) cells participate in a broad range of immunologic functions, including resistance to infection and anti-tumor immunity. NK cells are also the participants in the recently developed anti-cancer therapy that uses "lymphokine-activated killer" (LAK) cells (now in clinical trials). However, attempts to define the mechanisms by which NK cells are activated have been hampered by the difficulties in obtaining homogeneous, well-characterized populations of these cells. This proposal describes an in-depth evaluation of NK cells using cloned and characterized human NK cell lines. These stable NK cell lines exhibit defined, measurable phenotypic responses to cytokines, mediate normal cytotoxic functions, and grow in numbers sufficient for biochemical analysis. This NK cell model will therefore be used to determine how multiple ligand-receptor interactions can mediate or modify distinct intracellular events during NK cell activation. The principal focus will be on evaluating: 1) the interacting second messenger pathways that operate during NK cell-mediated cytotoxic reactions; 2) the ways in which selected extracellular cytokines and adhesion molecules modify the signalling system and subsequent functioning of NK cells. A major emphasis will be placed on identifying and characterizing the signals that mediate activation after an NK cell binds to a) NK-sensitive tumor targets or virally-infected cells (direct, non-MHC restricted, cell-mediated cytotoxicity); or b) Fc receptor-specific ligands (antibody-dependent cell- mediated cytotoxicity). Preliminary data has identified roles for five interrelated signal transduction pathways: 1) phosphoinositide turnover; 2) protein kinase C activation; 3) calcium signalling; 4)cAMP-dependent second messengers; and 5) eicosanoid metabolism. Recent experiments also suggest that IL-2, IL-4, TGF-beta, and cell adhesion molecules all have the capacity to modulate this NK cell activation. Further studies will be conducted to link signal transduction changes induced by these cytokines with their subsequent alterations in NK cell growth, differentiation, and function. Experiments designed to define the roles of these interacting factors will significantly enhance our understanding of both NK cell activation and its potential for therapeutic manipulation.